U.S. patent application number 10/128335 was filed with the patent office on 2002-10-10 for organic pigment and a method for its preparation.
Invention is credited to Lehtila, Reko, Malkki, Yrjo.
Application Number | 20020144629 10/128335 |
Document ID | / |
Family ID | 8555545 |
Filed Date | 2002-10-10 |
United States Patent
Application |
20020144629 |
Kind Code |
A1 |
Malkki, Yrjo ; et
al. |
October 10, 2002 |
Organic pigment and a method for its preparation
Abstract
The invention relates to a method for preparing an organic
pigment from starch, and the pigment thus received. According to
the invention, starch granules are swollen to increase their volume
and plasticity, their stability towards changes in volume and shape
is improved by cross-linking, by derivatization, or by making the
surface hydrophobic, after which gas bubbles or cavities are formed
inside the granules, these bubbles or cavities having a strong
light scattering effect. Generating bubbles or cavities can be
performed by evaporating water or another liquid, by releasing
impregnated gas, by a gas generating reaction, or by displacing
absorbed water with a solvent. The product is useful as a white
pigment especially in coating of starch, in paints and in cosmetic
products.
Inventors: |
Malkki, Yrjo; (Espoo,
FI) ; Lehtila, Reko; (Lohja, FI) |
Correspondence
Address: |
PENNIE & EDMONDS LLP
1667 K STREET NW
SUITE 1000
WASHINGTON
DC
20006
|
Family ID: |
8555545 |
Appl. No.: |
10/128335 |
Filed: |
April 24, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10128335 |
Apr 24, 2002 |
|
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|
PCT/FI00/00954 |
Feb 2, 2000 |
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Current U.S.
Class: |
106/217.01 |
Current CPC
Class: |
C09B 61/00 20130101;
C09B 67/0061 20130101 |
Class at
Publication: |
106/217.01 |
International
Class: |
C08L 003/00; C09D
103/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 2, 1999 |
FI |
19992365 |
Claims
1. A method for preparation of an organic pigment, characterized in
that the pigment is prepared by swelling starch granules in a
liquid phase, by stabilizing the granules in such a way that they
essentially maintain the outer dimensions which they have reached
in swelling, and by removing liquid from the granules in such a way
that light scattering gas bubbles or cavities filled with gas
remain in the granules.
2. A method according to claim 1, characterized in that during the
swelling the volume of the granules grows 2-4 fold from the
original volume.
3. A method according to claim 1 or 2, characterized in that the
swelling occurs in water below the gelatinization temperature of
the starch.
4. A method according to any one of the preceding claims,
characterized in that bubbles or cavities are generated by a rapid
evaporation of liquid present in the granule.
5. A method according to claim 4, characterized in that the
evaporation occurs by subjecting the granules to subatmospheric
pressure.
6. A method according to claim 4, characterized in that the
evaporation occurs by rapid heating of granules.
7. A method according to any one of the preceding claims,
characterized in that bubbles or cavities are formed by displacing
a liquid present in the granules with another liquid which is more
easily volatile, and by evaporating the last mentioned liquid.
8. A method according to claim 7, characterized in that the
granules are swollen in water, which is subsequently displaced with
methanol, ethanol ether and/or acetone.
9. A method according to any one of claims 1-3, characterized in
that bubbles or cavities are generated by releasing a gas
impregnated in the granules, or by effecting a gas releasing
reaction in the granules.
10. A method according to claim 9, characterized in that the gas is
carbon dioxide or a mixture of carbon dioxide and air.
11. A method according to any one of the preceding claims,
characterized in that granules are stabilized by cross-linking the
starch.
12. A method according to claim 11, characterized in that the
cross-linking chemical is glyoxal or epichlorohydrine.
13. A method according to claim 11 or 12, characterized in that the
cross-linking degree of starch is 0.5-6%, preferably 1-3%.
14. A method according to any one of claims 11-13, characterized in
that cross-linking is started before forming bubbles or cavities in
the granules.
15. A method according to claim 14, characterized in that granules
are added to a liquid phase containing cross-linking chemical in
such a way that swelling and cross-linking occur in the granules
simultaneously.
16. A method according to any one of the preceding claims,
characterized in that granules are stabilized with a reagent
increasing their hydrophobicity.
17. A method according to any one of the preceding claims,
characterized in that granules are stabilized by coating them with
a hydrophobic substance.
18. An organic pigment prepared by a method according to any one of
the preceding claims, characterized in that it contains swollen
starch granules which are stabilized in such a way, that when in
contact with water they substantially maintain their increased
outer dimensions, and which contain light scattering gas bubbles or
gas-filled cavities.
19. A pigment according to claim 18, characterized in that it is a
white pigment consisting of swollen and stabilized starch
granules.
20. A pigment according to claim 18 or 19, characterized in that
the dimensions of the bubbles or cavities are between 0.1-5
.mu.m.
21. A pigment according to claim 20, characterized in that the
cavities are longitudinal in shape, having a length principally
between 1-5 .mu.m, and a width principally between 0.1-0.8
.mu.m.
22. A pigment according to any one of claims 18-21, characterized
in that in a major part of the granules there are 1 to 10 light
scattering bubbles or cavities per granule.
23. A pigment according to any one of claims 18-22, characterized
in that at least the surface layer of the granules contains
cross-linked starch.
24. A pigment according to any one of claims 18-23, characterized
in that the surface of the granules has been made hydrophobic.
Description
[0001] The object of this invention is a new method to prepare
organic pigment and the new pigment obtained by this method. This
pigment is useful especially as a component for increasing
whiteness and opacity of various products.
[0002] Conventionally, pigments used for increasing whiteness and
opacity in paper coating, paints, cosmetic products and for
comparable purposes are composed of inorganic materials. Their use
impairs recycling of materials, because when the content of
pigments in a material exceeds certain limits, organic material
being the carrier or binder of the pigment cannot be burned without
a supporting fuel, or without other special arrangements, and the
material does not decompose biologically in dumping. Inorganic
pigments increase the gravity of the pigmented material and thus
freight costs of the final product. Some inorganic pigments contain
heavy metals and are thus not applicable in living environment.
[0003] Organic pigments have been developed for these purposes
mainly based on styrene-butadiene and urea-formaldehyde raw
materials, and they have been marketed as latex preparations. These
raw materials, too, are combined with environmental difficulties,
since they are not decomposed biologically, and for their safe and
innocuous burning, high temperatures are necessary. Some latexes
marketed function mainly as components giving gloss and without
affecting whiteness or opacity. As a white organic pigment, latex
composed of hollow particles of styrene-butadiene polymer, such as
the product ROPAQUE or Rohm & Haas company, has been marketed.
Light scattering of such particles are based on an air bubble in
the hollow space, the diameter of which is said to be 0.8 .mu.m.
Theoretical calculations have shown, that light scattering from air
or gas bubbles in an organic material is the strongest when the
diameter of the bubble is of the same order of magnitude as the
wavelength of light.
[0004] Of the renewable natural raw materials, starch, among
others, scatters in dry state light strongly and is sensed white.
As with other materials, light scattering is the stronger, the
finer the material, and thus stronger for the small-granular than
for large-granular starches. So far the common opinion has been
that light scattering occurs from the surfaces of the granules.
When starch is suspended in water or other liquid, light scattering
properties are significantly decreased.
[0005] When inorganic pigments are mixed in gelatinized starch, as
in paper coating paste, there is a difference on the interface in
the refractive index between the pigment and the binding material,
and thus light is either reflected or refracted depending on the
contact angle. When starch granules are mixed in starch, refractive
indices are the same or nearly the same on both sides of the
interface, and thus no reflection or refraction occurs on such an
interface. Starch granules cannot thus be used as such as pigments
in applications, where the binding material is starch, as it is in
paper coating. Their pigmenting effect is also weak when mixed in
organic liquids such as oils or solvents, due to a small difference
in the refractive index.
[0006] Starch and starchy materials have been swelled in several
industrial operations, for example in cooking extrusion and in
popping corn. In these operations, a starchy material containing
water is suddenly heated under pressure to temperatures above
100.degree. C., and the pressure is suddenly released, causing a
swelling of the material due to the water vapour generated.
However, at the temperatures and water contents used in these
operations starch is gelatinized. The magnitude of pores formed is
usually a few millimeters and thus not in the range optimal for
light scattering. Since starch in the walls of these bubbles is
gelatinized, the bubbles are not stable when in contact with
water.
[0007] In the method according to U.S. Pat. No. 5,925,380, one or
several thermoplastic synthetic monomers with ethene unsaturated
bonds are added in starch, and the mixture is heated at
temperatures where starch is not gelatinized. The said monomers are
polymerized forming hollow particles. Their content is 2-30% of the
final product; the particle size is 1-100 .mu.m, and the density in
general below 0.1 g cm.sup.-3. According to these figures, the
pores of the smallest particles could be in the size range of the
strong light scattering, but there is in the patent no mention of
light scattering properties.
[0008] Surprisingly it has now been observed, that dry starch
particles have sometimes brightly light scattering spots, where the
light scattering is manyfold as compared to the surface of a starch
granule. Such spots have been observed both in starch samples dried
rapidly using the so-called flash drying, and in slowly dried
starch samples. The light scattering spot is often in the amorphic
centre of the granule, but such spots seem to occur also on the
surface of granules. When the sample is contacted with water or
another liquid, the light scattering is weakened or disappears,
often irreversibely. Especially heating in the presence of large
amounts of water leads to disappearing of light-scattering spots.
In analogy with the said hollow organic pigments one can assume,
that the light scattering would be caused by air bubbles formed or
remaining in the granules or on their surface as the granules are
dried. For the irreversible disappearance or weakening of the light
scattering, two possible reasons seem to be evident. Firstly, when
the granules are moistened, the said hollow cavities or air bubbles
are filled with water, and material dissolved or suspended in water
fills these cavities. Secondly, when the granule is dried, it may
shrink in such a way that no new cavity is formed. Correspondingly,
heating in the presence of water effects gelatinization of starch,
and in this connection a disruption of the granule structure.
[0009] In the research on this invention it has now been found,
that it is possible to form in starch granules cavities or gas
bubbles, which cause a strong light scattering and are stable also
in contact with water and/or in short-time heating. The amount of
the cavities or bubbles can be significantly higher than what is
formed spontaneously in drying processes, thus resulting an
effective light scattering.
[0010] It can be calculated on the basis of the theoretical
knowledge on Light scattering, and also in analogy with other light
scattering particles, that light scattering of a cavity or air
bubble surrounded by starch is increased when its diameter is
decreasing. It has a maximum close to the wavelength of light.
Consequently, this phenomenon can be exploited under the following
preconditions: 1) by increasing the appearing of the bubbles or
cavities to a significant frequency, 2) by bringing the mean size
of the bubbles or cavities as close as possible to the wavelength
of light, 3) by reinforcing the walls of bubbles or cavities in a
way to maintain them gas-filled or prevent from collapsing also
when the starch granule is in contact with water, 4) by
concentrating the formation of bubbles or cavities as far as
possible close to the surface of the granules, where the intensity
of the incoming light is greatest. Starch granules when dry are
dense, and in part crystallized. A precondition for the formation
of bubbles or cavities is swelling in water, which also makes the
granule more plastic in its Theological behaviour. Unheated starch
granules can be swollen below the gelatinization temperature to a
2-3-fold volume or even more without altering the shape or
structure of the granule. It has been now found, that in the starch
plasticized in the said way, bubbles or cavities can be formed, for
instance, by 1) causing a liquid inside the granule or near to its
surface to evaporate rapidly, 2) by impregnating into the granule a
gas which is rapidly released from it, 3) with the aid of a
gas-evolving chemical reaction, or 4) by removing water imbibed
during swelling the granule by displacing it with a solvent. When a
liquid is evaporated or a gas is released slowly, only a minor
amount of bubbles or cavities are formed.
[0011] In a non-modified starch, bubbles or cavities formed are
easily collapsed when the granule is dried and shrinks to its
original volume. The collapsing can be however prevented by
stabilizing the granules while still swollen, in such a way that
the granules maintain the expanded outer dimensions.
[0012] According to the invention, a method is thus achieved for
preparing a new organic pigment from starch, based on chemical
and/or physical modification of starch. With the aid of these
modifications, strongly light scattering cavities or gas bubbles
are formed within starch granules, and these bubbles or cavities
will be preserved under application conditions of the pigments. In
addition, the invention includes a new starch-based pigment.
Essential characteristics of the invention are presented in the
Claims attached.
[0013] Stabilization of the granules can be successfully
implemented by cross-linking, using methods and reagents known as
such, for instance using glyoxal or epichlorohydrin. The degree of
cross-linking and its localization has to be optimized according to
the objectives. Especially in starch granules irregular in shape
and multiangular, such as oat starch, cross-linking is strongest at
the edges of the granule. When the granule dries, these edges
maintain their shapes and dimensions, while the less cross-linked
parts of the granule remain plastic, which leads during the drying
to a shrinking of the less cross-linked parts and drawing back
towards the centre. When the cross-linking is optimal in the entire
outer part of the granule, the outer shape and dimensions of the
swollen granule are maintained while drying, and in the interior
cavities are formed, the volume of which corresponds to the amount
of water removed. A high degree of cross-linking weakens the
plasticity of the starch, and bubbles or cavities are not formed
especially in the surface layers where the cross-linking is
highest. Cross-linking also elevates the gelatinization temperature
and thus improves the stability of the structure when heated.
[0014] Using transmission electron microscopy it has been verified,
that a part of the cavities arising are opened to the surface of
granule, and they have evidently functioned as pathways for
escaping of water vapour or gases. A part of the cavities do not
reach the granule surface, and thus they cannot be filled with
liquid when the granule is in a short contact with water, starch
paste or a solvent. The diameter of the cavities varies favourably
within the range 0.1-0.8 .mu.m, and their length within the range
1-5 .mu.m. The diameter is thus on the optimal range of light
scattering. It is to be expected, that at least the cavities which
remain closed have light scattering properties, but that due to
surface tension forces also the cavities opening to the surface
remain air-filled in water contacts, at least of short-duration,
and thus participate in the light scattering.
[0015] Maintaining the cavities air-filled can be improved by
treating the granules after the cavities or bubbles have been
formed with hydrophobic chemicals, for example by acetylating the
surface layer using acetic anhydride, by another derivatization
including graft copolymerization, or by coating the granules with a
thin layer of a hydrophobic chemical such as acetyl monoglyceride.
These alternative ways to stabilize granules can be used either
separately or for complementing the cross-linking treatment of
starch.
[0016] Cross-linking affects the formation and adds stability of
bubbles when the amount of the chemical is within the limits 0.5-5%
of the amount of starch. The degree of cross-linking of starch can
be 0.5-6%, optimally about 2-3%. Cross-linking can be performed in
acidic, neutral or alkaline conditions. The best results have been
obtained by treatments in alkaline conditions. For controlling
alkalinity, carbonates can be favourably used, this also enabling
evolution of gas when drying or under the influence of an acid.
Swelling before cross-lining is performed at temperatures below the
gelatinization temperature. Thus, for example, for oat starch, the
gelatinization of which starts at about 55.degree. C., the most
favourable swelling temperature is 45.degree. C. Swelling at a too
high temperature leads to a partial breakdown of the granules or to
damaging of their surfaces. Swelling and cross-linking can also be
performed simultaneously. When dry starch is added to water
containing a cross-lining reagent, a part of the chemical can
penetrate inside the starch granule through micropores present in
the granule, and the cross-linking can thus be more homogenous.
[0017] Generation of bubbles or cavities is most advantageous to
perform at a stage when starch is already partly cross-linked, but
still plastic enough for forming bubbles. Besides the degree of
cross-linking, plasticity is affected also by temperature. The
simplest way for forming bubbles is to evaporate water or other
solvent, such as ethanol, methanol, ether, or acetone present or
imbibed in the granules. This can be performed either by subjecting
the cross-linked starch material containing water or another
solvent to a subatmospheric pressure, or by elevating rapidly its
temperature, for instance in a drying equipment. Correspondingly,
bubbles can be formed from a chemical imbibed in the granules, such
as carbonates, by elevating the temperature, by changes of
pressure, or with the aid of acids. Formation of cavities is most
simply performed by swelling starch granules, cross-linking them or
stabilizing by derivatization including graft copolymerization, and
subsequently removing the water rapidly by drying or by displacing
it with another solvent.
[0018] Formation of bubbles or cavities can best be observed with
light microscopy performed by illuminating from the direction of
the objective. Bubbles and cavities are then observed as bright
spots with an apparent diameter of 0.5-1.5 .mu.m, but due to the
halo effect of the strong light scattering, the real diameter of
the largest bubbles cannot be exactly measured in light microscopy.
In scanning electron microscopy, only traces of broken bubbles on
the surface of granules have been observed. Their diameters have
been 0.3-1.5 .mu.m. Despite the bubble formation, the main part of
the granules have a smooth surface thus indicating that the bubbles
and cavities are in the deeper layers of the granules.
[0019] Starch granules are white in the native state and also after
being modified by means described above, and thus they form a white
pigment. The pigment can, however, be transformed by staining to
have another colour, according to needs of particular
applications.
[0020] The principles and implementation of the invention are
elucidated in the following examples. Examples 1 and 2 elucidate
the swelling of starch granules and formation of bubbles in the
granules. In the subsequent examples, stabilization of the granules
has been performed in addition. As the starting material, oat
starch has been used in the examples, but the method can also be
applied by using other starches as raw materials.
EXAMPLE 1
[0021] Oat starch was swollen by heating it in water at 60.degree.
C. during 12 minutes. In a microscopic examination using
illumination from the direction of the objective, the volume of
granules had grown to 3-4 fold from the original volume. Water was
displaced by 92% (weight per weight) ethanol, and ethanol with
ether, after which starch was dried at room temperature. In a
microscopic examination performed after ether had evaporated, 1 to
10 gas bubbles or cavities per granule were found. When such
granules were suspended in glycerol, light scattering disappeared,
and when suspended in oil, 1 to 3 bubbles were observed in more
than 50% of the granules. The size of the bubbles was 0.5-3 .mu.m,
the largest of them were longitudinal. For comparison, dry
non-treated oat starch was microscopically examined. In nearly each
granule, there was in the centre of the granule a cavity or a gas
bubble, which scattered light more intensively than the other parts
of the granule, but light scattering of all bubbles or cavities
disappeared after suspending in water.
EXAMPLE 2
[0022] The heat treatment described in Example 1 was repeated by
heating in water at 60.degree. C. for 5 minutes. By centrifugal
separation it was found, that 2.68 g water/g starch was bound.
Water was displaced with ethanol using two subsequent treatments.
After centrifugation, the ethanol content of the starch was 1.47
g/g. Ethanol was displaced by ether, and the sample was air dried
at room temperature. In microscopic examination immersed in oil,
nearly all granules had bubbles or cavities with a size of 0.5-3
.mu.m. Transmitted light darkened at these spots indicating that
light was reflected towards the direction of illumination. In
illumination from the sides, bubbles or cavities reflected light
brightly.
EXAMPLE 3
[0023] Oat starch was swollen by incubating it in water at
70.degree. C. for 5 minutes, and this was followed by cross-linking
by adding glyoxal, 1, 2, 3, 4, or 5% from the weight of starch.
Excess water was removed by centrifigation, and the damp sample
having a temperature of 60.degree. C. was subjected to vacuum
during 30 minutes. In microscopic examination using illumination
from the direction of objective, light scattering bubbles or
cavities were found in all of the samples treated. They were most
frequent in the sample with 3% cross-linking. In this sample, more
than 95% of the granules had 1 to 8 bubbles or cavities with
diameters from 0.3 to 0.8 .mu.m. When suspended in water, light
scattering was best preserved in the 3% cross-linked sample. In all
samples, even the darkened bubbles or cavities recovered, after
drying at room temperature, their light scattering ability to a
level which was superior to that of the starting material. The
light scattering ability was fully recovered, when the sample was
redried by displacing water with ethanol and ethanol with
ether.
EXAMPLE 4
[0024] 0.2 g of 3% cross-linked and vacuum treated starch prepared
according to Example 3 was mixed with 4 ml of acetic anhydride, 1
ml pyridine was added, and the mixing was continued at room
temperature for 19 hours. Starch was separated from the reagents by
centrifuging and washed three times with ether. The treatment
reduced the aggregation tendency of the granules. After contacting
with water and air drying, the light scattering ability of the
granules was maintained unaltered.
EXAMPLE 5
[0025] Cross-linking of 3% according to Example 3 was performed by
simultaneously leading a mixture of carbon dioxide and air into the
reaction vessel. Drying of the sample was performed under vacuum,
by intermittently leading the said gas mixture into the vessel, and
by repeating the vacuum treatment. In microscopic examination it
was found that leading the gas mixture increased the amount of gas
bubbles, their size and light scattering.
EXAMPLE 6
[0026] Stable air-filled light scattering cavities were formed in
starch granules by cross-liking it under alkaline conditions at
45.degree. C. with epichlorohydrin. The reaction was performed in
water phase by adding to the reaction mixture at 45.degree. C. and
pH 8.70, epichlorohydrin in an amount which was 2% of the amount of
starch. The reaction mixture was allowed to cool at room
temperature during 40 minutes, after which it had a pH of 9.1 and a
temperature of 23.4.degree. C. Water was removed from the mixture
by centrifugation. The product was air dried on glass plate, and
had already a significant amount of light-scattering cavities.
Light scattering was intensified when the damp sample was treated
in vacuum at 50.degree. C., or water was displaced by ethanol and
ethanol by ether, or by displacing water with acetone.
EXAMPLE 7
[0027] For improving water resistance of light scattering granules,
starch cross-linked to 2% by glyoxal and dried by ethanol and ether
treatments was mixed in a 10% solution of acetyl monoglyceride in
hexane, continuing the mixing under 5 minutes, and removing the
liquid by decanting. In the following microscopic examination of
the starch granules immersed in water it was found, that all
granules were coated with a hydrophobic layer of acetyl
monoglyceride. The light reflection of individual granules seemed
to remain unaltered, although the glyceride layer diminished the
total reflection observable. After drying the granules were found
to having remained intact under the contact with water.
* * * * *